在循环中设置进度视图或更新进度视图

Question 1

一个简单的解决方案是不同步地执行此操作，在进行时更新进度视图：

创建进度视图并将其添加到您的视图中
将您的代码派遣到背景队列
每次下载结束时，请将进度视图的更新派发回主队列

在伪代码中，看起来像

UIProgressView *progressView = [[UIProgressView alloc] init];
// configure the progress view and add it to your UI

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    for (int i=0; i<[array count]; i++)
    {
        NSError *error;
        NSArray *ipaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *idocumentsDir = [ipaths objectAtIndex:0];
        NSString *idataPath = [idocumentsDir stringByAppendingPathComponent:@"File"];
        NSLog(@"idataPath:%@",idataPath);

        //Create folder here
        if (![[NSFileManager defaultManager] fileExistsAtPath:idataPath])
        {
            [[NSFileManager defaultManager] createDirectoryAtPath:idataPath withIntermediateDirectories:NO attributes:nil error:&error];
        }
        // Image Download here
        NSString *fileName = [idataPath stringByAppendingFormat:@".jpg"];
        NSLog(@"imagePathDOWNLOAD:%@",fileName);

        _imgData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:[array objectAtIndex:i]]];
        [_imgData writeToFile:fileName atomically:YES];

        // now dispatch any UI updates back to the main queue
        dispatch_async(dispatch_get_main_queue(), ^{

            [progressView setProgress: (CGFloat) (i + 1.0) / [array count] animated:YES];
            tempImg.image = [UIImage imageWithData:_imgData];
        });
    }
});

也有一系列越来越优雅的方法：

使用并发队列（而不是上面下载图像串行下载）以下载图像，这将要快得多。我可能建议使用队列 maxConcurrentCount 的 5, ，享受并发，但请确保您不会超过并发请求数中的iOS限制。
利用 NSURLConnectionDataDelegate 基于下载而不是 NSData 方法 initWithContentsOfURL, ，这可以在单个下载过程中提供临时进度。看下载管理器或者下载操作举些例子。
利用 afnetworking 它还提供下载基于块的界面。

在上面，在第1点，我建议您考虑使用并发队列，因此我决定对其进行基准测试。对我来说，下面的GCD实现比 NSOperationQueue 遵循它的实施。

这是GCD实施：

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

UIProgressView *progressView = [self addProgressView];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    NSInteger downloadSuccessCount = 0;
    NSInteger downloadFailureCount = 0;

    NSString *idataPath = [self createDownloadPath];

    for (int i = 0; i < [array count]; i++)
    {
        // Image Download here
        NSString *filename = [self pathForItem:i array:array folder:idataPath];
        NSURL *url = [self urlForItem:i array:array];
        NSData *data = [[NSData alloc] initWithContentsOfURL:url];
        UIImage *image = nil;
        if (data)
            image = [UIImage imageWithData:data];
        if (image) {
            downloadSuccessCount++;
            [data writeToFile:filename atomically:YES];
        } else {
            downloadFailureCount++;
        }

        // now dispatch any UI updates back to the main queue
        dispatch_async(dispatch_get_main_queue(), ^{

            [progressView setProgress: (CGFloat) (downloadSuccessCount + downloadFailureCount) / [array count] animated:YES];

            // update the image in the UI if you want

            [UIView transitionWithView:self.imageView duration:0.25 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
                tempImg.image = image;
            } completion:nil];
        });
    }

    NSLog(@"Completed in %.1f seconds", CFAbsoluteTimeGetCurrent() - start);
});

对此 NSOperationQueue 执行：

CFAbsoluteTime start = CFAbsoluteTimeGetCurrent();

UIProgressView *progressView = [self addProgressView];

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 5;

NSString *idataPath = [self createDownloadPath];
self.downloadSuccessCount = 0;
self.downloadFailureCount = 0;

NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"Completed in %.1f seconds", CFAbsoluteTimeGetCurrent() - start);
}];

for (int i = 0; i < [array count]; i++)
{
    NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        // Image Download here
        NSString *filename = [self pathForItem:i array:array folder:idataPath];
        NSURL *url = [self urlForItem:i array:array];
        NSData *data = [NSData dataWithContentsOfURL:url];
        UIImage *image = nil;
        if (data)
            image = [UIImage imageWithData:data];
        if (image)
            [data writeToFile:filename atomically:YES];

        // now dispatch any UI updates back to the main queue
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            if (image) {
                self.downloadSuccessCount++;

                // update the image in the UI if you want, though this slows it down

                [UIView transitionWithView:self.imageView duration:0.25 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
                    tempImg.image = image;
                } completion:nil];
            }
            else
                self.downloadFailureCount++;

            [progressView setProgress: (CGFloat) (self.downloadSuccessCount + self.downloadFailureCount) / [array count] animated:YES];
        }];
    }];

    [queue addOperation:operation];
    [completionOperation addDependency:operation];
}

[queue addOperation:completionOperation];

底线，如果您使用 NSOperationQueue （不仅提供并发性，您还可以在GCD并发队列中进行，而且还可以使您轻松地控制并发操作的数量（网络操作应限制为五个或更少）），您将享受一个绩效的重大好处。

正如我建议的那样，更好的是使用Afnetworking，在这种情况下，您不仅可以享受此操作队列并发利益，而且还享受其他两个好处。

Question 2

[progressView setProgress: (CGFloat) (i + 1.0) / [array count] animated:YES];
self.progressLabel.text = [NSString stringWithFormat:@"%.0f",self.progressView.progress*100];